Stroke is a leading cause of long-term disability in the United States and Canada and the third leading cause of death after heart disease and cancer. Many epidemiologic studies have found an association between short-term increases in particulate air pollution and cardiovascular morbidity and mortality. Exposure to particulate matter may also increase the risk of ischemic and/or hemorrhagic stroke, but this hypothesis has only been examined using data from administrative records or death certificates. By conducting a case-crossover study using prospectively collected research-quality data from >16,000 patients hospitalized between 2003 and 2008 with acute stroke in 12 medical centers across Ontario, Canada, we aim to test the following hypotheses: 1) Short-term increases in ambient levels of particulate matter can precipitate the onset of acute ischemic and hemorrhagic stroke;2) the induction time from ambient particulate matter exposure to the onset of stroke symptoms will be quantifiable after very short-term exposures (hours) and after intermediate-term exposure (days);3) patients with cardiovascular co-morbid conditions including type II diabetes, previous stroke and coronary artery disease will be most susceptible to these effects. Patient data will be obtained from the population-based Registry of the Canadian Stroke Network (RCSN). Individuals'exposures to ambient particulate matter and other ambient pollutants will be assessed using postal code of residence and hourly monitoring data from air quality monitors located in the catchment areas of participating hospitals. By better quantifying the relationship between ambient air pollution and specific stroke subtypes and by identifying susceptible populations, the results of this project may inform the planning of future mechanistic studies and may have implications for stroke prevention. This application offers several advantages over previous studies, including information on the timing of stroke symptom onset and important clinical covariates in a very large number of unselected patients with acute stroke (ischemic and hemorrhagic). Moreover, this application reflects a new collaboration with investigators from the Canadian Stroke Network and lays the groundwork for future collaborative projects. PUBLIC HEALTH RELEVANCE: Stroke is a leading cause of long-term disability in the United States and the third leading cause of death after heart disease and cancer. There is some evidence that short-term increases in particulate air pollution may trigger the onset of stroke in some people. In this study, we will evaluate 16,000 patients with stroke to evaluate the effect of air pollution as a potential trigger of stroke onset.